1. Field of the Invention
This invention relates to magnetic tape recording, and more particularly to apparatus and methods for laying out data on magnetic tape.
2. Background of the Invention
In some tape drives, such as current LTO and enterprise-level tape drives, variable-length blocks of data are received from a host interface and converted into fixed-size units known as data sets. These data sets are typically broken into smaller fixed-size units known as sub data sets (SDSs). Error correction coding is then generated for each SDS as a unit to protect the data contained therein.
To protect the data in an SDS, conventional tape drives may organize the data in an SDS into a two-dimensional array of rows and columns. Each row in the two-dimensional array may be made up of multiple (e.g., 2 or 4) interleaved data words. Error correction codes (i.e., ECC codes) may then be generated for each row in the array and each column in the array to protect the data contained therein. This in essence provides two dimensions of protection for data in the array since protection is provided for both rows and columns. Once generated, the error correction codes may be appended to the array for eventual storage on the magnetic tape medium. Each row of the extended SDS array (which includes the appended ECC codes) may be referred to as a codeword interleave (CWI), the likes of which will be described in more detail hereafter.
In order to ensure the number of errors in an SDS do not overpower the ECC codes used to protect the SDS, the rows of the SDS may be physically distributed (i.e., laid out) on the magnetic tape in such a manner that, if errors occur spatially close to one another on the magnetic tape medium, the errors will be spread across multiple SDSs in the data set. This will ideally “decorrelate” error locations on the magnetic tape from error locations within an SDS. Stated otherwise, the rows of each SDS may be physically distributed on the magnetic tape medium with the intent of limiting the number of errors that occur in any one SDS. Limiting the number of errors occurring in an SDS increases the probability that the ECC parity associated with the SDS will be powerful enough to correct the errors contained therein.
The tape layout schemes used in five generations of LTO drives (i.e., LTO 1, 2, 3, 4, and 5) employ various combinations of track rotation, codeword interleave (CWI) set swaps, and track swaps in order to evenly distribute CWIs on the tape and thereby “decorrelate” error locations on the tape from error locations within each SDS. One drawback associated with these tape layout schemes is that they do not optimize the distances between CWIs to achieve an optimum degree of decorrelation. Thus, a tape layout design is needed that substantially maximizes the distances between the CWIs of each SDS, while evening out CWI set and track distribution, in order to increase decoding reliability.